Magnetic Resonance Imaging (MRI) was introduced in the 1980s, and has developed into a major global imaging modality with current sales of approximately 3,000 scanners worldwide per annum.
Clinical MRI depends for its success on the generation of strong and pure magnetic fields. A major specification of the static field in MRI is that it has to be substantially homogeneous over a predetermined region, known in the art as the “diameter spherical imaging volume” or “dsv.” Errors or variation of less than 20 parts per million peak-to-peak (or 10 parts per million rms) are typically required for the dsv.
MRI equipment has undergone a number of refinements since the introduction of the first closed cylindrical systems. In particular, improvements have occurred in quality/resolution of images through improved signal to noise ratios and introduction of high and ultra high field magnets. Improved resolution of images, in turn, has led to MRI being a modality of choice for an increasing number of specialists for both structural anatomical and functional human MRI imaging.
The basic components of a typical magnetic resonance system for producing diagnostic images for human studies include a main magnet (usually a superconducting magnet which produces the substantially homogeneous magnetic field (the B0 field) in the dsv), one or more sets of shim coils, a set of gradient coils, and one or more RF coils. Discussions of MRI, can be found in, for example, Haacke et al., Magnetic Resonance Imaging: Physical Principles and Sequence Design, John Wiley & Sons, Inc., New York, 1999. See also Crozier et al., U.S. Pat. No. 5,818,319, Crozier et al., U.S. Pat. No. 6,140,900, Crozier et al., U.S. Pat. No. 6,700,468, Dorri et al., U.S. Pat. No. 5,396,207, Dorri et al., U.S. Pat. No. 5,416,415, Knuttel et al., U.S. Pat. No. 5,646,532, and Laskaris et al., U.S. Pat. No. 5,801,609, the contents of which are incorporated herein in their entireties.
Conventional medical MRI magnets, including cryostat and covers, are typically around 1.6-2.0 meters in length with free bore diameters in the range of 0.8-1.0 meters. Normally, the magnet is symmetric, and the midpoint of the dsv is located at the geometric center of the magnet's structure. The uniformity of the axial component of the magnetic field in the dsv is often analyzed by a spherical harmonic expansion.
The typical aperture available to accommodate a patient in a conventional MRI machine (i.e. inside the gradients and RF transmitter coil) is a cylindrical space having a diameter of about 0.6-0.8 meters, i.e., just large enough to accept the subject's shoulders, and a length of about 2.0 meters or more. The dsv for such systems is located near the center of the aperture, which means that it is typically about a meter from the end of the aperture.
Not surprisingly, many people suffer from claustrophobia when placed in such a space. Although there have been improvements in patient comfort through the introduction of open systems in the early 1990s, and short bore high field closed systems in the early 2000s, there is still a distinct need for smaller magnetic resonance systems in modern medical imaging.
In addition to its effects on the subject, the size of the magnet is a primary factor in determining the cost of an MRI machine, as well as the costs involved in the siting of such a machine. Standard 1.5 T MRI whole body scanners, due to their size, weight, fringe field and power needs, demand highly specialised and expensive infrastructure before they can be installed, including development of separate multi-room imaging suites. These requirements mean that in most cases, only larger hospitals or substantial imaging clinics can afford to install such systems and offer MRI as a diagnostic modality to patients.
In order to be used safely, MRI machines often need to be shielded so that the magnetic fields surrounding the machine at the location of the operator are below regulatory agency-specified exposure levels. By means of shielding, the operator can be safely sited much closer to the magnet than in an unshielded system. Longer magnets require more shielding and larger shielded rooms for such safe usage, thus leading to higher costs.
Extremity MRI (which, for the purposes of this application, is also called orthopedic MRI) is one of the growth areas of the MRI industry, with 20% of all MRI procedures in the United States in 2006 being performed on upper extremities (e.g., arms, wrists, and elbows) and lower extremities (e.g., legs, ankles, and knees) (IMV, 2007). This equates to 5.3 million extremity procedures in 2006, compared with around 110,000 in 1990, when extremity scans made up only 2% of total MRI procedures.
Extremity MRI systems are much smaller than whole-body or conventional MRI systems and are much easier to site, due both to their reduced size and reduced stray fields. They are therefore a low cost solution to the imaging of extremities. As discussed below, extremity imaging is a particularly preferred application for the magnets of the present invention.
While extremity MRI systems have a number of advantages to the subject and the operator, they represent a challenge in terms of the space available for the various coils making up the magnet and in terms of cooling those superconducting coils. A major difficulty in realizing a superconducting magnet is to produce a large imaging dsv (of the required homogeneity) when the magnet length is reduced, while ensuring the superconducting wires can be used safety and efficiently.
Open systems, which comprise the larger portion of dedicated extremity systems, are constrained by being limited to lower field strengths; the highest field open MRI scanner on the market in 2005 was the Philips 1.0 T system.
The low field nature of the current smaller MRI systems on offer is a major disadvantage to their use. According to the American College of Rheumatology, ‘the low-field MRI systems are unable to obtain the SNR of high-field MRI systems for images of similar spatial resolution’. Low field systems generally have longer image acquisition times, which can be problematic for procedures requiring contrast agents, since for extremity procedures, intravenously injected contrast agents can diffuse into the joint fluid in a period of minutes.
The present invention is directed to providing improved magnets and magnetic resonance systems which address these and other challenges of extremity MRI systems.